Electrochemical machining process and electrolyte composition of chloride and sulfates



United States Patent 3,401,103 ELECTROCHEMICAL MACHINING PROCESS AND ELECTROLYTE COMPOSITION OF CHLORIDE AND SULFATES Alexander H. Joyce, Detroit, and Lawrence V. Puls, Madison Heights, MiclL, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Oct. 23, 1965, Ser. No. 504,109 12 Claims. (Cl. 204-143) This invention relates to electrochemical machining processes and more particularly to electrolytes for use therewith.

In recent years electrolytic machining procedures for generating shaped cavities and contoured surfaces have been developed and are generally classified into one of two basic categories, namely electrolytic grinding and electrochemical machining. The electrolytic grinding process was developed early in the 1950s to reduce the requirement for cutting and grinding cemented carbides and is essentially an electrochemical deplating process which can be used on any electrically conductive material. It is generally not adapted to cavity sinking operations but rather to metal removal operations performed by cutoff wheels, saws and grinding or milling machines. Electrolytic grinding, or electrolytic assisted grinding, as the process was originally called, uses equipment similar in appearance to conventional grinders except for the electrical accessories. About 95% of the metal removal results from electrolytic rather than mechanical action. One version of the electrolytic grinding process is char acterized by a rotating cathode containing abrasive particles, an essentially neutral electrolyte and a static or mildly agitated bath of electrolyte. Electric current is passed through the bath dissolving the anodic surface and the resultant film of insoluble salts is then scraped away by the rotating grinding wheel. Electrochemical machining, on the other hand, relies on reaction product removal by means of electrochemical action and is characterized by a plunging action of the tool electrode and a rapid circulation of the electrolyte. Generally then, the former process requires mechanical action for reaction product removal, i.e., a grinding wheel, and the latter provides for said removal by an initial dissolution of the reaction products at the workpiece-electrolyte interface, a subsequent hydroxyl precipitation of the metal ion, and a final flushing away of the precipitate resulting from the flow of electrolyte through the workpiece-to-electrode gap.

While aqueous solutions of individual inorganic salts, such as nitrates, nitrites and others, have been used as electrolytes in electrochemical machining processes, aqueous sodium chloride solutions appear to be best suited for general application work and are therefore most commonly used today. However, regardless of what salt is used, an inherent problem with the electrochemical machining processes utilizing a solution of but one salt or a non-critical mixture of salts is the production of articles presenting undesirable surface characteristics and/ or smut which clings tenaciously to the equipment and workpiece.

It is therefore an object of this invention to produce a smut free, accurately electrochemically machined article having desirable machined surfaces.

It is a further object of this invention to effect these results by utilizing an electrolyte comprising a solution of selected salts in a fixed proportion which varies as a function of the workpiece composition so as to provide a uniform dissolution rate of the grains and grain boundaries of the respective material.

It is a further object of this invention to produce the desired results by utilizing in an electrochemical machining process an electrolyte comprising a solution of chlorides, sulfates and nitrates in a balanced relationship with- 3,401,103 Patented Sept. 10, 1968 in fixed limits with the particular concentration configuration being variably dependent upon the composition of the workpiece.

Further objects and advantages of the present invention will become apparent from the following detailed description of the invention.

We have found that the aforementioned undesirable surface characteristics fall mainly into two categories, the first being of a granular leather-like nature resulting from the preferential dissolution of the grain boundaries of the particular alloy and the second being the inverse of the first, in that there is a preferential dissolution of the grains themselves leaving projections of grain boundary alloy composition extending above the machined surface. These undissolved grain boundaries, which remain as thin ridges and needle-like projections surrounding the cavities left by the dissolved grains, have little mechanical strength and are easily dislodged, resulting in the production of dark smut.

A specific embodiment of my invention involves the machining of a zinc base die casting alloy SAE 903 utilizing a preferred electroyle comprising 12 oz. per gallon of sodium chloride, 15 oz. per gallon of sodium sulfate and 3.5 oz. per gallon of potassium nitrate. However, prior to the machining using the aforementioned preferred electrolyte, similar alloy samples were machined utilizing electrolytes comprising first 8 oz. per gallon of potassium chloride and 8 oz. per gallon of sodium sulfate, and sec- 0nd primarily nitrates respectively. A microscopic exam ination of the surface resulting from the machining utilizing the former electrolyte (chlorides and sulfates) revealed that the chloride ion had preferentially attacked the grain boundaries, which are essentially a solid solution of 96% aluminum and 4% zinc. While the actual function of the sulfate ion is not known, it appears to anodize the aluminum alloy while attacking the predominantly zinc grains, which effectively modifies the effects of the chloride electrolyte, thereby producing a smut free but granular electrochemically machined surface. The sample produced by a machining utilizing the latter electrolyte (nitrate) showed indications that the nitrate had preferentially attacked the grains themselves, which are approximately 99.7% zinc, leaving the grain boundaries remaining as thin ridges and needle-like projections surrounding the cavities left by the dissolved grains.

Combining the two electrolytes into the aforementioned preferred proportions yielded a balanced electroylte having the property of being able to dissolve both the grains and the grain boundaries at the same rate, hence producing a uniformly smooth machined surface. This particular electroylte therefore effects an even metal removal rate on a zinc die casting alloy by balancing the preferential grain attack of sulfate and nitrate ions against the preferential grain boundary attack of the chloride ion, resulting in a uniform erosion rate across the surface of the workpiece,

While a preferred electrolyte for the machining of an SAE 903 zinc die casting alloy is 12 oz. per gallon of sodium chloride, 15 oz. per gallon of sodium sulfate and 3.5 oz. per gallon of sodium nitrate, the balance being water, or, more generally, one equivalent of sulfate ion per equivalent of chloride ion and 0.2 equivalent of nitrate ion per equivalent of chloride ion, the following range of concentration ratios have produced smut free surfaces: 0.20 to 3.0 equivalents of sulfate ion per equivalent of chloride ion and 0.1 to 0.40 equivalents of nitrate ion per equivalent of chloride ion.

It should be noted that the use of nitrate ions limits the operating temperature of the bath to less than F. Above this temperature nitrate ions are reduced, thereby decreasing the efficiency and producing objectionable ammonia fumes. Increasing the temperatures increases the action of the nitrate ion more than the action of the chloride ion, causing a roughened Surface in a low current density area due to the preferential grain removal at those points. Therefore, the preferred electrolyte operate best within the temperature range of from 100 to 120 F. However, low temperatures ranging from 100 F. down to room temperature have produced satisfactory surfaces. It has been found that for temperatures in excess of 120 F. but less than 140 F. it is best to reduce the nitrate concentration from to 25%, depending on the specific temperature used.

With a given electrolyte at a given temperature, it is necessary to increase the electric potential per unit anode to cathode distance to obtain an increase in current density. These higher electropotentials decrease the effect of half cell potentials and the activation energies of the constituents of a heterogeneous alloy, permitting optimum metal removal over a wider range of anion ratios and higher current densities. For low current density removal such as is encountered with stray currents and certain stationary electrode applications, there is a requirement for closer control of the anion ratios. While this particular electrolyte was developed at relatively low current densities, it has satisfactorily been used at current densities ranging from 4 amperes per square inch to 4,000 amperes per square inch.

While we have found that nitrate free electroyltes with a sulfate ion to chloride ion ratio of 1.1 to 1.0 will produce a smut free surface at all current densities above 5 amperes per square inch, the nitrate rich electrolyte is still preferred to permit maximum flexibility over a broad range of operating conditions using but a single basic electrolyte.

We have found that the cations have little or no effect on the quality of the surface produced by the electrochemical machining. In fact, in applications of multiple anion electrolytes higher concentrations can be used without precipitation due to the common ion effect of using more than one cation. However, the lighter alkali metal (i.e. Li, Na, & K) salts are generally preferred because they produce relatively neutral pI-Is, do not plate out or have a deleterious effect upon the cathode, and represent a source of inexpensive material.

It has also been noted that in these multi-component electrolytes it is the ratio of the anions one to the other that determines the resulting machined surface and that this ratio is of greater importance than the overall concentration of the respective anions. Aqueous sodium chloride solutions have been used ranging from very dilute to saturation, but we have found that it is impractical to operate under conditions where the concentration is less than 16 oz. per gallon of sodium chloride. At the lower concentrations, solution conductivity and metal removal rates are seriously affected.

We have also found that these same three anions can be used to compound electrolytes suitable for the electrochemical machining of cast iron, cold rolled steel, magnesium, aluminum, zinc and alloys thereof.

The following are specific examples of successful experiments encompassed within the scope of our invention:

Example No. 1

A bath comprising 8 oz. per gallon of sodium chloride, 8 oz. per gallon of sodium sulfate and 2 oz. per gallon of potassium nitrate was used to machine a sample of SAE 903 zinc alloy at a current density of 30 amperes per square inch and an electrolyte temperature of 80 F. The

electrolyte was maintained at a pressure of 20 lbs. per square inch and flowed at a rate of 30 gal. per minute through the workpiece-to-electrode gap, which was initially maintained at 0.015 inch. A metal removal rate of 0.005 inch per minute was experimented over a period of two minutes and the finished sample exhibited a smooth and smut free surface.

4 Example No. 2

A bath comprising 16 oz. per gallon of sodium chloride, 8 oz. per gallon of sodium sulfate and 4 oz. per gallon of potassium nitrate was used for twelve seconds to machine samples of SAE 903 zinc alloy at a current density of 1900 amperes per square inch and an electrolyte temperature of 110 F. The electrolyte was maintained at a pressure of 100 lbs. per square inch and flowed at 100 ft. per second through the workpiece-to-electrode gap, which was initially maintained at 0.010 inch. The electrode was constantly fed toward the workpiece at a rate of 0.350 inch per minute. An electropotential of 18 volts was maintained. The finished samples exhibited a bright electropolished surface having a very thin line of light smut at the edges of the part nest. However, a chemical analysis of this smut showed that it was a resultant of copper impurities in the materials used and not undissolved grain boundaries. While generally excellent detail in terms of reproducing the surface of the electrode was found under all operating conditions, the best detail was found to be produced under conditions of the more rapid electrode feed rates. This particular solution was satisfactorily operated at current densities from 500 to 1900 amperes per square inch, electrolyte pressures from 40 to 100 lbs. per square inch and flow rates from 20 to 100 ft. per second.

Example No. 3

A bath comprising 16 oz. per gallon of sodium chloride, 4 oz. per gallon of sodium sulfate and 8 oz. per gallon of sodium nitrate was used for forty seconds to machine a sample of SAE 903 zinc alloy at a current density of 500 amperes per square inch and an electrolyte temperature of 110 E, which electrolyte was maintained at a pressure of 40 lbs. per square inch. The electrode was fed toward the workpiece at a rate of 0.060 inch per minute for a total feed of 0.050 inch with an initial workpiece-to-electrode gap of 0.010 inch. The surface of the workpiece was smooth and bright in the immediate cut area, but displayed a slightly roughened surface in overcut areas.

Example No. 4

A bath comprising 4 oz. per gallon of sodium chloride and 12 oz. per gallon of sodium sulfate was used for two minutes to machine a sample of SAE 903 zinc alloy at a current density of 30 amperes per square inch and an electrolyte temperature of R, which electrolyte was maintained at a pressure of 20 lbs. per square inch. The electrolyte flowed through the workpiece-to-electrode gap of 0.015 inch at a rate of 30 gallons per minute. A metal removal rate of 0.005 inch per minute was experienced andthe finished sample exhibited a smooth, smut free surface.

Example No. 5

A bath comprising 8 oz. per gallon of sodium chloride, 8 oz. per gallon of sodium sulfate and 6 oz. per gallon of potassium nitrate was used for two minutes to machine a sample of SAE 903 zinc alloy at a current density of 30 amperes per square inch and an electrolyte temperature of 80 E, which electrolyte was maintained at a pressure of 20 lbs. per square inch. The electrolyte flowed through the workpiece-to-electrode gap of 0.015 at a rate of 30 gallons per minute. A metal removal rate of 0.0055 inch per minute was experienced and the finished sample exhibited a smooth, smut free surface.

While current densities ranging from to 2,000 amperes per square inch would cover most practical electrochemical machining applications, electrolytes having compositions in the preferred ranges have been used with current densities as low as 4 amperes per square inch, and it is at these low current densities that the effect of the different electrolyte anions becomes most apparent.

We have found that the subject electrolyte will permit an electrode feed rate of up to 0.750 inch per minute, but in order to maintain such an electrode feed rate it is necessary to increase the current density to approximately 4,000 amperes per square inch. Even under these rather severe operating conditions, the electrolyte performed adequately.

While the invention has been described in terms of certain preferred embodimments, it is to be understood that others may be adopted and the scope of the invention is not limited thereby except by the following claims.

We claim: 1. A process for electrochemically machining a metal consisting essentially of the steps of establishing said metal as the anode in an electrochemical cell, orienting a cathodic electrode adjacent to but closely spaced from said metal so as to form a gap therebetween, flowing through said gap an aqueous electrolyte consisting essentially of a balanced mixture of ionizing salts of chlorides, sulfates and nitrates which erode the grains and grain boundaries of said metal at a uniform rate, and passing current through said metal, electrolyte and electrode, whereby the surface of said metal is uniformly electrolytically eroded, wherein the concentration of said sulfate is from 0.20 to 3.0 equivalents of sulfate per equivalent of chloride and the concentration of said nitrate is from 0.1 to 0.40 equivalent of nitrate per equivalent of chloride.

2. A process as claimed in claim 1 wherein said chlorides, sulfates and nitrates are salts of the alkali metals.

3. A process as claimed in claim 2 wherein said metal is from the group consisting of: iron, magnesium, zinc, aluminum and alloys thereof.

4. A process as claimed in claim 3 wherein said metal is iron and its alloys.

5. A process as claimed in claim 4 wherein said alloy is steel.

6. A process as claimed in claim 3 wherein said metal is zinc and its alloys.

7. A process as claimed in claim 6 wherein said alloy is a zinc-aluminum alloy.

8. A process for electrochemically machining a metal selected from the group consisting of iron, magnesium, zinc, aluminum and alloys thereof consisting essentially of the steps of establishing said metal as the anode in an electrochemical cell, orienting a cathodic electrode adjacent to but closely spaced from said metal so as to form a gap therebetween, flowing through said gap an aqueous electrolyte consisting essentially of chlorides and sulfates wherein the concentration of said sulfate is from 1.0 to 1.1 equivalents of sulfate per equivalent of chloride, and passing current through said metal, electrolyte and electrode.

9. The process according to claim 8 wherein said chlorides and sulfates are salts of the alkali metals.

10. The process according to claim 8 wherein said alloy is steel.

11. The process according to claim. 9 wherein one of said alloy is a Zinc-aluminum alloy.

12. An aqueous electrochemical machining electrolyte consisting essentially of alkali metal chlorides, sulfates and nitrates wherein the concentration of said sulfate ion is from 0.20 to 3.0 equivalents of sulfate ion per equivalent of chloride ion and the concentration of said nitrate ion is from 0.1 to 0.40 equivalents of nitrate per equivalent of chloride.

References Cited Report No. l\/lAl2--l8-M, prepared under Contract DA-49-025-SC-83 between Dept. of Defense and the Nat. Acad. of Sciences, January 1952.

ROBERT K. MIHALEK, Primary Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.0. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,401,103 September 10, 1968 Alexander H. Joyce et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 13 and column ()1, lines 8 and 9, "consisting essentially of", each occurrence, shctld read comprising Signed and sealed this 24th day of February 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. A PROCESS FOR ELECTROCHEMICALLY MACHINING A METAL CONSISTING ESSENTIALLY OF THE STEPS OF ESTABLISHING SAID METAL AS THE ANODE IN AN ELECTROCHEMICAL CELL, ORIENTING A CATHODIC ELECTRODE ADJACENT TO BUT CLOSELY SPACED FROM SAID METAL SO AS TO FORM A GAP THEREBETWEEN, FLOWING THROUGH SAID GAP AN AQUEOUS ELECTROLYTE CONSISTING ESSENTIALLY OF A BALANCED MIXTURE OF IONIZING SALTS OF CHLORIDES, SULFATES AND NITRATES WHICH ERODE THE GRAINS AND GRAIN BOUNDARIES OF SAID METAL AT A UNIFORM RATE, AND PASSING CURRENT THROUGH SAID METAL, ELECTROLYTE AND ELECTRODE, WHEREBY THE SURFACE OF SAID METAL IS UNIFORMLY ELECTROLYTICALLY ERODED, WHEREIN THE CONCENTRATION OF SAID SULFATE IS FROM 0.20 TO 3.0 EQUIVALENTS OF SULFATE PER EQUIVALENT OF CHLORIDE AND THE CONCENTRATION OF SAID NITRATE IF FROM 0.1 TO 0.40 EQUIVALENT TO NITRATE PER EQUIVALENT OF CHLORIDE. 